Thick film covercoat materials are generally used to electrically, mechanically and hermetically insulate a thick film circuit. A wide variety of materials exist at various costs for different applications. Inexpensive glass covercoats are often employed for their hermeticity properties usually to improve resistor stability. Organic covercoats provide some electrical insulation and often are used for mechanical protection of the circuit during low temperature processing. Dielectric materials are used for the electrical insulating properties.
However, during the refiring process of two-sided thick film circuits, damage may occur to the underside conductor runners that come into contact with a furnace belt. Common glass covercoats flow and stick to the belt upon refiring. Organic covercoats cannot withstand the high temperatures of the refiring process. Dielectric materials often discolor in the areas closest to the belt mesh and are expensive to use. The problem of preventing damage to the conductive runners on the underside of a two-sided thick film circuit may be solved by propping up the substrate so that the circuitry does not touch the belt during the refiring process. However, this is time consuming and usually is done manually. The problem may also be solved by modifying the belt furnace. However, this is very expensive and may result in limiting the flexibility of the furnace to run various products.
In order to form a two-sided thick film circuitry semiconductor device, a conductive runner composition is printed on the top face of the substrate. The substrate is then fired in a furnace to cure the printed conductor runners. A covercoat is then applied to the top face circuitry and fired. The substrate then may be flipped over and the process repeated for the bottom face. However, upon the refiring process of the two-sided thick film circuits, damage may occur to the underside conductive runners that come into contact with the furnace belt if the overcoat is not capable of withstanding high temperatures.
Thus, there is a need for a thick film covercoat which mechanically protects a two-sided thick film circuitry substrate and which is capable of withstanding the high temperatures of the refiring process.